1. Field of the Invention
This invention relates to thin film solid state electrochemical cells. More particularly, this invention relates to novel stabilized negative electrodes for electrochemical cells, consisting of a lithium metal coated with a thin film of an electroactive polymer capable of transmitting alkali metal ions interposed between the lithium anode and the polymer electrolyte.
2. Prior Art
Rechargeable lithium-polymer batteries are promising advanced power sources for a variety of applications such as to portable electronic devices and electric vehicles. Although a variety of polymer electrolytes have been investigated as the ionic conducting medium, the reactivity of lithium metal has posed a formidable challenge to develop polymer electrolytes which have the requisite chemical and electrochemical stability for long cycle life [F. M. Gray, Solid Polymer Electrolytes (VCH Publishers, Inc., New York 1991)].
A key problem with lithium metal anodes is the formation of dendrites upon repeated plating of lithium metal during charging of the battery. This has led to a detailed investigation of lithium alloys, such as lithium-aluminum alloys, and lithium-carbon composites as alternatives to lithium metal anodes. U.S. Pat. No. 4,002,492 discloses electrochemical cells having anodes consisting of lithium-aluminum alloys where the content of lithium is from 63% to 92%. Other disclosures of lithium-aluminum anodes are found in Rao, et al., J. Electrochem. Soc. 124, 1490 (1977), and Besenhard, J. Electrochem. Soc., 94, 77 (1978). The use of lithium-carbon composite anodes is disclosed in Ozawa et al., in Proc. Tenth International Seminar on Primary and Secondary Battery Technology, Deerfield Beach, Fla., March 1993.
The central problem with composite lithium anodes is an increase in weight and volume due to the addition of non-electroactive materials. In the case of lithium-aluminum alloys, there is also a loss in potential of about 0.4 V. The loss in cell voltage coupled with increased weight implies a significant loss in specific energy of the cell. Batteries using lithium-aluminum alloys as anodes have exhibited relatively low capacities, low rate capabilities and poor cycle life.
Lithium-carbon composites based on intercalation in graphitic carbon generally have a voltage drop of 0.3 V-0.5 V vs. lithium and typically involve 8-10 carbon atoms for each lithium atom, the theoretical maximum being 6 carbon atoms for each lithium atom. This entails a significant penalty in increased weight and volume, and, consequently, decreased capacity. Cells using lithium-carbon composite anodes have, however, demonstrated long cycle life, with more than 1,000 cycles recorded.
Shacklette, et al., disclose the use of a conjugated polymer-lithium composite anode in U.S. Pat. No. 4,695,521, which incorporates an n-doped conjugated polymer as a substrate for electroplating a lithium metal, resulting in finely divided lithium metal distributed throughout a conducting polymer matrix. Cells incorporating conjugated polymer-lithium composite anodes have long cycle life, but reduced capacity. The n-doped conjugated polymers have low capacity that limits the capacity of the anode material.
Toyoguchi, et al., disclose the use of a prefabricated film of a conjugated polymer to coat the lithium surface of an anode in a cell using a liquid organic electrolyte in kokai 58-163188 (1983). Cells with lithium anodes coated with a conjugated polymer showed enhanced cycling ability compared with equivalent cells using bare lithium anodes. Prefabricated conjugated polymer films are highly porous and at least 10 micrometers thick in order to have sufficient mechanical strength to be free-standing. Porous films are not suitable if the electrolyte is polymeric since a polymer electrolyte cannot penetrate the pores of the film, resulting in inferior contact between the conjugated polymer and the electrolyte. With liquid organic electrolyte, a porous film does not provide complete surface coverage, and therefore not as complete protection as a dense polymer film. The relatively thick prefabricated conjugated polymer films also add significant weight and volume to the cell, limiting the capacity of the cell.
There is a clearly defined need, therefore, for novel concepts in interfacial engineering of the lithium-electrolyte interface that allows the fabrication of rechargeable lithium cells having long cycle life and incorporating polymer electrolytes.